Unfortunately, at this time it is not possible to customize the default meeting invitation text. If you are using Outlook to send your meeting invites, you could save the messaging as a signature in Outlook that you could then insert into the meeting invitation with a couple of clicks.
I apologize this feature request has not been implimented.
The default invitation template cannot be customized for automatic distribution, but the text can be customized to anything you want, after it populates into the Email. We provide the template with a nice join link + dial in info, and you can change this formatting to something else once the email populates. If you have an email signature on a localized email application, this should also populate into the message triggered by GoToMeeting.